I just played someone who had an unbeatable loop with 2 cards, which rendered the whole game unplayable. I was winning significantly, but a 2 card combo just kept looping through over and over until I had then lost and he had won. This seems like it shouldn't be possible - it's definitely a game breaking loop.

The 2 card combo is:
Starscape Cleric
Bloodthirsty Conqueror.

I can only imagine there are more combos which would do the same thing, but essentially it loops through:

1. Any creature attacking the player
2. Bloodthirsty Conqueror causing the opponent to gain that much life
3. Starscape Cleric causing me to lose life, due to the opponent gaining life
4. Steps 2 and 3 repeat infinitely, causing an unbreakable loop of damaging me and healing the opponent over and over and over again until I lose.

Seems a super broken combo and may want to be looked at.